1. Field of the Invention
The present invention relates generally to heat exchangers and more particularly to support structures for heat exchanger tubes within heat exchanger devices.
2. Background of the Invention
Although heat exchangers were developed many decades ago, they continue to be extremely useful in many applications requiring heat transfer. While many improvements to the basic design available in the twentieth century have been made, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes.
In particular, one of the most problematic aspects associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the various deposits and coatings which form on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased performance with respect to heat transfer. Fouling also causes an increased pressure drop in connection with the fluid flowing on the inside of the exchanger.
Many heat exchangers in use today also contain baffles. Baffles are interposed in the fluid path in order to ensure that the fluid flowing on the outside the tubes flows across the tubes. Unfortunately, however, baffles serve to increase the fouling problem because they create dead zones on the shell side of the exchanger.
One type of heat exchanger which is commonly used in connection with commercial processes is the shell-and-tube exchanger. In this format, the device is designed such that one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bundle in a serpentine fashion.
Fouling can be decreased through the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. It is known that the use of higher fluid velocities can substantially decrease or even eliminate the fouling problem. Unfortunately, higher fluid velocities are generally unattainable on the shell side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system because of the baffles.
Another problem that often arises in connection with the use of heat exchangers is tube vibration damage. Tube vibration is most intense and damage is most likely to occur in cross flow implementations where fluids flow is perpendicular to the tubes, although tube vibration damage can also occur in non-crossflow (i.e. axial) implementations in the case of very high fluid velocities.
Existing shell-and-tube heat exchangers suffer from the fact that they must typically use baffles to maintain the required heat transfer. This, however, results in “dead zones” within the heat exchanger where flow is minimal or even non-existent. These dead zones generally lead to excessive fouling. Other types of heat exchangers may or may not employ baffles. If they do, the same increased fouling problem exists. Further, in heat exchangers fitted with baffles, for example, the cross flow implementation results in the additional problem of potential damage to tubes as a result of flow-induced vibration. In the case of such damage, processes must often be interrupted or shut down in order to perform costly and time consuming repairs to the device.